ABSTRACT Higher animals and plants undergo a process of developmental maturation following embryogenesis. This process involves simultaneous changes in many different organs and tissues, which must be temporally coordinated if the organism is to develop properly. Coordination between the maturation of non-reproductive organs and the maturation of the reproductive system is particularly important because variation in the relative timing of these processes can have serious consequences for the health and well-being of an organism, and for its ability to reproduce. During its development, a plant shoot transitions from a juvenile to an adult pattern of vegetative development (vegetative phase change), and becomes competent to produce flowers. In Arabidopsis thaliana, vegetative phase change is regulated by a decrease in the level of miR156 and the consequent increase in the expression its targets, SPL transcription factors, which promote adult vegetative traits. It is thought that this vegetative transition is linked to the acquisition of reproductive competence through the effect of SPL proteins on the expression of MIR172B. However, there is evidence that reproductive competence may actually be regulated by transcription factors that operate in parallel to the MIR156 -| SPL -> MIR172B pathway. This hypothesis will be tested by examining the effect of mutations in these transcription factors on the ability of the shoot to respond to a brief floral inductive stimulus. Recent results suggest that the evolutionarily conserved chromatin remodeler, PICKLE, (PKL) is a key component of the mechanism that coordinates the timing of vegetative phase change and reproductive competence. PKL promotes the transcriptional repression of MIR156A/C by promoting the deposition of the repressive histone mark, H3K27me3, at these genes. The molecular mechanism of this process will be investigated by determining if PKL promotes histone deacetylation, and if it promotes the binding of proteins involved in the deposition of H3K37me3 to MIR156A/C. Site-directed mutagenesis will be used to identify sequences required for the temporal expression of MIR156C. Finally, the degree to which the timing of vegetative phase change and reproductive competence vary naturally will be determined by measuring these traits in 150 genome-sequenced accessions of A. thaliana from around the world. Loci responsible for variation in the timing of vegetative phase change will be mapped in a subset of these accessions using Traffic Lines. These experiments will answer long-standing questions about the genetic regulation of shoot maturation, and will provide new insights into the molecular mechanism of this process and its role in the evolution of A. thaliana.